Bilateral modulator circuit



March 14, 1933. E. PETERSON BILATERAL MODULATOR CIRCUIT Filed March' 21, 1929 2 Sheets-SheetI l A T THNE Y March 14, 1933. E; PETERSON 1,901,408

BILATERAL MODULATOR CIRCUIT Filed March 21, 1929 2 SlfleelLS-Sl'leel 2 /NVEN Tof? l E. PE TERso/v www A TTM/vir Patented Mar. 1,4, 1933 UNiTED STATES PATENT orties EUGENE PETERSON, OP NEW YORK, N. Y., AssIGNORTo WESTERN ELECTRIC COMPANY, INCORPORATED, or NEW YORK, N. Y.,A CORPORATEON OE NEW YORK BILATERAL MODULATOR CIRCUIT Application inea March ai,"

rier side band waves and for `conversely translating incoming signal bearing carrier side band waves into the corresponding signal waves. .Y i

ln order that these two types of frequency translation may be performed in' a single device the related circuits from which waves having the respective frequencies are impressed on such device must necessarily be in close physical proximity. ln fact, they may more or less completely overlap since certain practical forms of bilateral modulators effectively have only two terminals so that the input and output circuits of the modulator, or from another point'of view, the signal and side band impressing circuits, are in part common. The operation of bilateral modulator'circuits therefore introduces hazards notl found in the more'conventional alternative circuit arrangements in which the corresponding circuits are more nearly physically distinct. The component parts of a bilateralcircuit tend to react on each other so as to impair their respective functions as well as to promote singing and cause various types of interference.

It is an object of this invention to provide circuit arrangements for bilateral modulator circuits which combine lrelatively great elhciency, simplicity and lowl costwith adequate protection against interference andv` singing, both of which tend to be incidental to the operation of such circuits.

This Object as well as other related and incidental objects may be realized in part by the use of frequency discriminating networks. In a particular type of bilateral modulator circuit that is preferred for some purposes, the modulatorelement is itself unilaterally conducting, so that the modulating currents for outgoing transmission and the incoming side band currentstraverse the modulator inthe same direction.V For this case-to provide protection from singing, in addition to tlie protection afforded as `unilateral modulator element.

19,29.' serial No. 348,740.

above .by frequency discrimination (selec- Specifiare caused to traverse in part a common Circuit element from whioha potential is derived for impressing on the input Ofjsuch ant modulation products, that is, the carrierv side band or the reproduced signal as the case y may be, are fed back through a reentrant circuit tosuch element and in such manner as to traverse electricalequivalent portions of it and its associated circuits in opposite directions andtherefore by'means ofy one or the other The result y most` fundamentalaspect Vto that used in a repeateri circuit which has become .well l.known as the Ql-type' repeater. cally,ithe modulating and side band currents of such portions to its destined terminus where it is to be usefully employed. The relation of the feed back circuit to such element, if an adequate balance is secured, is

lsuch that there is no resultant potential n impressed from suc-h feed-back circuit on the input of the modulator which if present Awould tend tofpromote singing.

The invention will be more fully described in connection with the accompanying drawings, wherein: y v

` Fig. 1 is a diagrammatic representation of a two-way carrier communication system employing a bilateral modulator of the inl vention at each terminal;

Fig. 2 is a diagrammatic representation of a 21type repeater vcircuit which maybe considered as a prototype of theV two-way carrier circuit of Fig. l; and

Figs. 3 and l disclose modified forms of the bilateral modulator of Fig. lcharacterized by the use of theprineiple of carrier suppression. y

In the circuit ofk Fig. 2 signalcurrents to be amplified may be derived from a circuit not shown connected to conductors l and flow in a circuit which includes the four line conductor windings of the balanced'network "or hybrid coil represented generally by reference numeral 2. This current induces an electromotive force in the remaining windings of the balanced circuit, which electroinotive force or corresponding current is amplified in amplifier or repeater 4 and ieimpressed on the line through circuit 5. rllhe mode of connection of circuit 5 to the line is such that the currents derived from said circuit and flowing in 'opposite directions in the line through the respective pairs of line windings of the hybrid coil effect a balance of magneto-motive forcesy in the input coils 3 of the repeater so that amplification may occur as above, without regeneration or singing. The amplified current flows in conductors 6 which constitutes an extension of conductors 1, pairs of conductors 1 and 6 constituting the complete transmission line. The currents incoming from the opposite direction, .that is, currents derived from circuits, not shown,

which are connected to conductor G, are

VVtransmitted through the hybrid coil and amplifier in exactly the same way as has been described with reference to the currents derived from conductors 1.

The arrangement of Fig. 2 therefore constitutes a two-way transmission circuit employing a single non-singing amplifying de-v vice, and the device 4l with its immediately associated circuits may be considered as a bilateral amplifier Yor repeater, although the amplifying or repeating element per se is unilaterally conducting. The system, when used as above, that is, as an amplifier rather than as a frequency translating fmeans as in the invention to be next described, is well known in the communication art as -a 21-type repeater.' It is described in detail in a paper by Gherardi and Jewett in the Transactions of the American circuit arrangements atteach terminus of Y mission line may equally well beadapted F i0'. 2 simulate in their o eration those of .C P Fig. 1. For example, it may be considered as a system like that of Fig. 2 modified by the replacement of the amplifier element by a modulator-demodulator, the system otherwise being modified as necessitated by the frequency translation which attends modulation. Reference numeral 7 represents a 'transmission line connecting terminal stations disclosed at the right and left. EachV terminal station involves apparatus and circuits for translating relatively low frequency signal waves into carrier modulated signal side bands, the transmission line accordingly ,being adapted to transmit such high frequency currents. The transfor simultaneously transmitting low freuenc currents incieaendence of 'function of the high frequency and low frequency circuits contemplated by such composite use being insured by a high pass filter HP2 inserted in the high frequeny circuits iinmediately adjacent the transmission line at either terminal, and by corresponding low pass filters LP similarly inserted in low frequency lines 8 connected to the transmission line.

The two terminals of the two-way system are identical, although the showing in the drawing of the iight terminal is made more diagrammatic than the left terminal. The remainder of thedescription of Fig. 1 will accordingly be confined to the left terminal, the labeling of the parts described being duplitcataed at the right terminal except where made impossible on account of 'the Vdiagrammatic showing thereof. i

These circuits may be best described by an analysis of their method of operation havingv in mind the functional relationship of theA circuits to the 21-type repeater circuit of Fig. 2 above described; Signal waves, later destined to modulate a carrier wave to produce the transmitted side-bands are derived from circuit 9 and are impressed .through transformer 10, which may be dispensed with on occasion, through low pass filter LP2 on the modulator represented generally by M.

The circuit from low pass filter LP2 through themodulator may be traced as follows: Upper conductor of circuit 11, the right hand conductor of circuit 1 2, the lower of the two input coils 13 of the modulator, the conductor 24, coil 23, ground 2G, and by the left hand conductor of circuit 12 back to filter LP2. A potential derived from these signal waves is impressed on the modulator on account of thecoupling between the lower coil 13 and lower coil 25. Carrier current for the modulator is impressed onl the upper input coil 13 of the modulator from its source C through circuit 17, amplifier A, if amplification is desired, transformer 18, high pass filter HP1, and circuits 15 and 14, in Vthat order.V The circuit for the carrier current is completed from the upper input coil 13 through conductor 24, coil 23 and ground 26. By analogy with theconditions affecting the operation of an ordinary hybrid coil arrangement current is unable to flow in either direction through the net- Vwork comprising coils 13 and 25 between cir- Vally suppress both the carrier and modulating currentsand keep them from flowing in transmission line 7. f

The filters y.LP LPZ, HP1 and HP2 may conform in' type and fimction to filters conlventionallyused to perform similar specific ilo functions. Examples of filters which may be used in the system of Fig. 2 are illustrated in U. S. Patent 1,227,113 to Campbell, May 22, 1917. Similarly the carrier source is not specific to this system but may Vbe adapted from conventional practice.` F or example, the source may be a vacuum tube oscillator of the type disclosed in the U. S. patent to Hartley 1,356,7' 63, October 26, 1920.

The input circuits of thermionic electric discharge devices 20 and 21, comprisedl in the modulator M, are related to each other and to their immediately associated circuits in a manner quite similar to that of the well known push-'pull'amplifien as described for example, in U. S. patent to Colpitts, 1,128,- 292, February 16, 1915. However, theV out,- put circuits of the two devices are somewhat dierently related than in the circuit of that patent, in that the output wave is derived from a. common portion of the anodecathode circuits of the two devices instead of from a circuit differentiallyrelated to the devices. It results from the particular relation of ele-ments disclosed that any wave vimpressed onV theV modulator circuit from coil and therefore impressed differentially en the interna-l input circuits of the devices mutually compensate or balance each other in lthe common output coil 22 and are therefore not transmitted to the external output circuit connected tok coil 23 whichis coupled to' said output coil.

The same is true of powers of suchv ,impressed waves and odd order modulated side bands, that is, side bands whose frequencies a-e indicated Aby expressions like 2P- I-Q,

`ilo

iso

2QiP, fPiQ and QiP, in which Pand Q represent the carrier and modulator frequencies.

On the other hand, harmonic components of the impressed waves, resulting from modulation, and alsoevenorder slide vbands such as are indicated byl expressions like PiQ, 2P2Q, etc., are not balanced out but occur in coil 22 and are transmitted through coupled coil 23.

As a practical matter, the only output products of themodulator which 'are utilized are second order side bands. They are transmitted from the terminals of coil 23, through conductor 24 to limpress a differlence of potential between groundand the input circuit to the modulator. Theconnection to the input circuit is at the midpoint of the coils 13 so that the two currents flowing in relatively opposite direct-lions from the point of connection, if equal, cause equal electromotive forces to be induced in coils 25 coupled with coils 13, so that by the circuit relationshlips'so far pointed outmodulation of the carrier by the signal Wave may he effected without regeneration'or singing on account of the modulator circuits. The resistances shown 1n shuntto certain of the :have to have low coils are `approximately equal sto vthe imped-` ances ofsuch coils. The use lof such resistances has `been found to -improve the fre= quency characteristics ofthe coils. f. Although a single thermionic discharg device would be adequatelto form the requisite lmodulatingfunctions, the combination of two such devices in push-pull relation'is necessary to insure :a condition of balance and therefore an avoidance ofv singing. @i5 Equality of the currents in coils 13 is insured by the electricalsymmetry of the paths of current flow in' either direction `.to

ground 26 from the point ofconnection be,-

tween such coils.V rPhat is, the filters LP1 and HP1 with 'their immediately 'associated terminating circuits have the samey impedancelcharacteristics as 'the similar combina- Y tion of liltersLPgandiHPz and their'immediately associated circuits. 5 In the particular modulator organization above described, the circuits containing the filters LP1 and HP1, and similarlyg'in the case of filters LP.2 andHP2 have their tery minating elements `attire ,modulator ter-V minalsfin parallel. I -n order-'thatthes'encircuits, including their `filter networks, maybe substantially functionally independent, the terminating networkelemen'tsf on the `sides toward the modulator should have highm-F pedance `outside the transmission. ranges of the filters. If the relation of these Vfilters to keach other were such that the'lterterminations were presented to thel modulator 'in series as might well be the case in an 'alternasigo' tive. arrangement, thesev terminationsr would 'impedance outside of the transmission ranges.- f

With respect to the complemental function ofthe modulator, that is, .with respect e toits use 'as a demodulator of incoming. side bands `to reproduce faV signal, the operation .is identically the Vsame so far asconcerns the modulator per se andr its immediately Y associated circuits, the'currentl to be com- 1.110 bined with the carrier in thisv case being derl rivedfrom the -transmission line Vand being ,transmitted to circuit -12 ythrough trans- Y former 19 and filter HP instead of yfrom the circuits atV the left of points of circuit 12. A Y

The system may-be adapted for .single or double side band transmission, without qualitication of the inherent'principle of the invention or its method of operation, by-proper' 120 ldesign of the high frequencyyiilters HP1 and HP2. Although, of coursevvith respect to the 'transmitted'wave the filter HP2 is solely responsible for thel choice fof: frequencies transmitted, `any modification of thisv lter, as to determine whether av singlegside band Vorboth side Ybands-.shouldbe transmitted,

should be duplicated flltenHPl -toiobserve the essential symmetryg.- v Figs. 3 and .4 illustrate slightlygvariant. 130

the connection; 115,

forms of a bilateral modulator'system of a novel type, alternative tothat illustrated in Fig. 1. Although these modulators and their immediately associated circuits are not illustrated as adapted without change for substitution for modulator M in the system of Fig. l, it is obvious that as far as regards their method of operation, they are equally well adaptedv for use therein and could be so used with obvious modifications.

The circuits of Figs. 3 and 4 differ essentially from each other only in 'that Fig. 3 illustrates a direct coupling of the input circuits to the modulator devices per se, in contradistinction to the inductive coupling through two-winding transformers of Fig. 4, and similarly functioning elements are similarly labeled. Fig. 4V shows in detail a practicable design of filter circuits that may be used in a system of that figure or in the system of Fig. 3`wherein the similar elements lare indicated diagrammatically only.

In the type of bilateral modulator disclosed in Figs. 3 and-4 carrier suppression occurs on account of the symmetrical relation of coil 27, by means of which the carrier current derives from circuit 28 is impressed on the two electric discharge elements 29 and 30 of the modulators, with respect to such elements. lThe signal and modulation side bands are each impressed in the same way on the modulators and differentially with respect to its two electric discharge elements. The low frequency signal currents are transmittedthrough low pass filter LPS, the vmodulation side bands being similarly transmitted through filter HP3 The circuits of Figs. 3 and 4 differ,l other than by the particular means for carrier suppression, from the modulator of Fig. l in that they'use two-element modulator devices per se. This'considerablyV simplifies the circuit and avoids thejnecessity for the vreentrant connection, as in the modulator .of Fig. l.

TheV operation of the circuits ofFigs. 3 and '4 alsoV differs from that of Fig. l in the modulation products which are balanced out. The presentlconsidered circuits balance out, in addition tothe carrier, the even harmonics of the impressed signal and side bands but permit to flow in the output circuit both even order side bands, and odd order side bands of the type represented by 2PQ, etc., that is, the types of odd order side bands that would be most useful in practice. For a complete treat- "ment of high order modulation, especially third order modulation, reference is made to U. S. Patent to Peterson,1,678,163, granted July 24, 1928. c.

By reason Vof this diversity of function, the systems of Figs. 3 and 4 may be alternately used, for example, for either the more conventional type ofimodulation, here denominated thesecond order', or the most desirable tyipe `of od'd order modulation, that is, third order modulation. If third order modulation is used, of course, the filters HPS must be adaptable for a much wider range of transmission than for the other case.

The novel features inherent in the invention are pointed out in the accompanying claims.

What is claimed is:

l. In combination, a transmission line, means for impressing relatively low frequency waves on said line, means for impressing carrier waves on said line, a modulating device, circuit means connecting the input circuit of said modulator to said line, and circuit means connecting the out-put circuitfof said modulator to said line contiguous to said input circuit connecting means, said input andk output connecting circuits being conjugately related 'so that output energy produces substantially no difference of potential between-the input terminals of the modulator device.

2. A modulator circuit comprising a low frequency source, a carrier frequency source,

Va modulator, a common terminal circuit for impressing the waves from said sources and for deriving the waves from said modulator, and means preventing 4regenerative amplii cation in said modulator. v

3. A two-way low frequency line, a twoway high frequency line connected thereto, modulating means having its input circuit connected to said lines at their `junction points and its output circuit also connected said input circuit.

4. In combination, a transmission line, means for impressing relatively low frcquencies on said line, means for impressing carrier waves on said line, a modulatingdemodulating device comprising an input circuit, a circuit connectingfsaid input cir-.

cuit to said line, and a circut for impressing output currents of said device' on said line, said last mentioned circuit comprising one connection directly to one side of the line and a-second connection to a point in said input circuit such that current flowing therefrom in opposite directions in said input circuit to the point of direct connection vproduces a balance' of output electromotive forces, whereby said deviceis made non-regenerative with respect to said output currents. A n

5. A combinationdefined by claim 4 comprising additionally, a frequency discriininating network in said line on either side of said connecting circuit and adapted respecto said lines but Vconjugately with respect to Yno tively to transmit the low frequencies and 'and similar networks inthe portion of said connecting circuit on the opposite side of said input circuit from said line whereby the output current flowing inthe two tions of the input circuit and'through the two pairs of frequency discriminating networks encounter impedances of like characteristics.

6. A combinationV defined by claim 4 comprising additionally, a frequency discriminating network in said line on either side of said connecting circuit and adapted to4 line, a two-way high frequency line con- Y nected therewith7 frequency discriminating networks in the respective low frequency and high frequency lines, a two-terminal modulator-demodulator device having an input and output circuit, means connecting one terminal of said input circuit directly to said lines at one of theirrjunction points, means for connecting the other terminal of said lines at the other junction point through a balancing circuit adapted to simulate the impedance characteristics of said lines andv their included networks, means for limpressing carrier waves on said high frequency line of said device, and means associated with said device whereby itsoutput circuit is connected to said lines both directly and through said balancing circuit but conjugately with respect to said input circuit.

8. In a modulator-demodulator in combiv nation, two similar two-element 'electric discharge modulator-demodulator devices each comprising a cathode and anode, a circuit connecting the cath`ode and anode of each said device and comprising an individual portion and a portion common to both said connecting circuits, means associated with said common portion for impressing a carrier wave of like phase on the anodes of said devices, and means associated with the individual portions for, Vin common, impressing" modulating and modulated waves differentially on said anodes and 4for similarly deriving the resultanty modulated and reproduced modulating waves from said devices.

In witness whereof, I hereunto subscribe my name this 20th day of March, 1929.

EUGENE PETERSON.

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